CAP

12/21/2001

CAP: Carrierless Amplitude and Phase modulation. A modulation technique used for DSL. CAP is a proprietary technology from Globespan semiconductor. CAP actually uses carriers - 113.2KHz uplink and 435.5, 631 or 787.5 downlink. Bandwidths are 136K uplink and 340/680/952 downlink. CAP claims to have a low peak to average signal power ratio that simplifies hardware design by requiring lower dynamic range from the circuitry.

Proponents of an alternate modulation scheme -- DMT -- claim that there are often substantial interoperability problems between CAP implementations from different suppliers.

In principle, CAP is similar to the analog QAM modulation scheme used in analog modems. Two carriers 90 degrees out of phase are combined at the transmitter and separated at the receiver thereby doubling the information capacity of a given amount of bandwidth. In general, QAM uses analog techniques to handle the quadrature signals whereas CAP uses digital techniques.